Armadillo Repeat Only Proteins Are Crucial for the Function of Plant CNGC Channels

Plant cells respond to a wide range of stimuli through intracellular calcium (Ca ²⁺ ) signaling. Cyclic nucleotide-gated channels (CNGCs) are a major class of plant Ca ²⁺ channels, with 20 homologs in Arabidopsis . These tetrameric plasma membrane (PM) proteins are activated by diverse stimuli, such as phytohormones, extracellular damage and cell wall integrity signals. Here, we identify a novel class of plant-specific proteins, Armadillo Repeat Only (ARO), as essential regulators of possibly all plant CNGC channels. Abrogation of functional sporophytic AROs results in a phenotypic pattern reminiscent of CNGC dysfunction, including defects in root gravitropism, root hair growth and morphology, stomatal movement, and responses to extracellular ATP and the phytohormone auxin. aro2/3/4 mutants are fully resistant to the toxic effects caused by overexpression of CNGCs. AROs colocalize and physically interact with multiple CNGCs and modulate CNGC-dependent currents in Xenopus oocytes. Structural modeling suggests that AROs form a tetramer surrounding the CNGC channel. These findings highlight AROs as key plant-specific players in plant CNGC function and Ca ²⁺ signaling.